Hybrid circuits and multichip modules are commonly used to connect electronic components for applications in instruments, computers, telecommunication equipment and consumer electronic products which require higher density and performance than the capabilities of printed-circuit boards. Typically, an engineer will design a hybrid circuit or a multichip module to carry the types of electronic components (including integrated circuits, transistors, and discrete components such as resistors, capacitors and inductors) necessary to implement the desired electronic function and to fit in the space available for the product. Consequently, each hybrid circuit or multichip module typically is custom designed. To design a custom hybrid circuit or a multichip module is expensive, takes time and requires custom tooling and the fabrication of prototype interconnect substrates. If errors are found in the prototypes, then the interconnect substrate must be redesigned. Such a process often delays the planned introduction of a new product. Bare dice, surface-mounted packages and electronic components are used with hybrid circuits and multichip modules, to provide high density. Shorter connection traces result in lower capacitances to drive shorter signal propagation delays, and higher performance. Testing of the integrity of the interconnects and the electronic components in the hybrid circuits and multichip modules is rather difficult. Attaching the probes of oscilloscopes and logic analyzers to observe the waveforms on the various pins of the electronic components during operation requires microprobing of fine lines and pads which is difficult. Many of the interconnects to be tested are imbedded and difficult to test. The use of test pads to access some of the imbedded traces takes area and often unintentionally overlooks important traces. The testing, diagnosis and debugging of hybrid circuits and multichip modules are complicated, time consuming and often delay the planned product introduction.
In the prior art, universal interconnect substrates which can be programmed to provide any desired interconnection pattern are described in U.S. Pat. No. 4,458,297; No. 4,467,400; No. 4,487,737 and No. 4,479,088. The universal substrates in the prior art used interconnect architecture which could not be programmed or tested from external leads and which required probing of internal pads to program the connections or to test the integrity of the interconnects or the electronic components on the substrate. To optimize the number of internal pads for the programming and testing of the interconnects, prior art architecture has interconnects with excessive lengths and parasitic capacitances which reduced the speed of the connections on the substrates. The difficulties in testing and programming such universal substrates are time consuming and often increase the product development time and expenses. Other prior art (an article entitled "Active Substrate System Integration" by Wooley et al, at the Center for Integrated Systems at Stanford University, copyrighted 1987 by the IEEE) discloses the potential use of active circuits in the substrate to implement drivers, receivers, repeaters and power distribution circuits. Such circuits are custom designed for each case and are active during the operation of the electronic components and chips on the substrate to implement the desired function.
I disclose in my U.S. patent application Ser. No. 07/410,194 filed Sep. 20, 1989, entitled "Field Programmable Printed Circuit Board", now U.S. Pat. No. 5,377,124, a printed circuit board of unique configuration combined with one or more special programmable integrated circuit chips (often called "programmable interconnect chips" or "PICs") to provide a user programmable printed circuit board capable of being used to provide any one of a plurality of functions. The active circuits in the programmable interconnect chips also provide test ports which offer powerful structures for testing the integrity of the interconnects, the electronic components and the system function.
The field programmable printed circuit board described in the above application substantially reduces the cost associated with developing complex electronic systems by providing a standard PC board configuration which is easily and economically manufactured. As disclosed in the above application, the designer of electronic systems utilizing the standard programmable PC board described therein will also utilize computer aided design software to determine the optimum placement of the electronic components on the programmable PC board and to determine the configuration of the programmable interconnect chip or chips to properly interconnect the electronic components so as to yield the desired electronic system.
In the present application, interconnect substrates for hybrid circuits and multichip modules with circuits in or mounted on the substrate are disclosed. These circuits enable the engineer by using external leads to electrically program the interconnects in the field and/or to connect any sets of nodes on the substrate to external test ports to test the integrity of the interconnects and the electronic components on the substrate and the system function. These circuits provide the benefits of the field programmable printed-circuit boards for applications requiring the density and performance of multichip modules and hybrid circuits. During normal operation these circuits in or mounted on the substrate can be disabled and the connections of the interconnects on the substrate provide the desired function.